It is known that the increase in speed on railway tracks, which can now be as high as 270 km/h and will soon be 300 km/h, or even higher, has led to the creation of very long switches to allow very high speeds at deviations, of the order of 170 km/h to 220 km/h and even higher.
In these very long switches, the crossings are also very long and, at these speeds, it is known that it is preferable to use crossing frogs with a moving point, which are more comfortable than crossing frogs with a fixed point, in which the counter-rails cause return movements of the vehicle axles, which are difficult to tolerate.
Indeed, crossing frogs with a moving point have been constructed for many years, but current designs are not completely satisfactory
Crossing frogs with a moving point generally comprise two fundamental elements:
the cradle in which the moving point is fixed at the heel and in which it moves when maneuvered;
the moving point itself.
The cradle is necessarily composed of several elements because their respective length is limited by the current state of production methods. It is therefore necessary, for producing the desired construction, to manufacture several elements, then to join them to form the cradle. As it is impossible, for constructional reasons associated, in particular, with a lack of space, to produce this joint by conventional fish-plating which would hold the elements of the cradle at the interior and the exterior, C-shaped members placed at the exterior of the cradle elements are used to form the joint. Now such a joint does not have the qualities of fish-plating and, even less, the qualities of a weld and, however much care is taken when producing and joining these C-shaped parts, this joint will necessarily exhibit problems in strength in the long term.
Furthermore, the moving point, produced from machined and joined rails, is secured at its heel in one of the cradle elements in the manner of a bracket embedded in a wall.
Its flexibility therefore enables it to rest against one of the point elements of the cradle in the position at the right or at the left when it is maneuvered by appropriate devices.
As these switches are incorporated in long welded rails, that is to say are connected to framing rails without expansion device, the fixing of the moving point has to ensure - apart from the role of embedding the bracket - that the compressive or tensile stresses of each long welded rail are transferred from the moving point onto the cradle.
In the conventional manner, such stresses are transferred by fish-plates stuck to the rail and to the frog.
Now when a molded embedding cradle element is used, the adhesion of the parts allowing the point to be secured in the cradle can obviously be produced on the moving point side where there is a rail form but cannot be produced on the cradle side as it has the form of a rigid box.
It is therefore necessary, to ensure that the stresses are transferred, to use a mechanical joint which has to be produced with high precision to reduce the clearances and to comply with the constraints for incorporation into long welded rails.
With current constructions, there are two mechanical adjustments which are difficult to make, are liable to deteriorate during service and thus have a detrimental effect on the service life of the product.